A light guide (otherwise known as a waveguide) traps light by providing total internal reflection with the light guide. This is achieved by controlling the angles over which light enters the light guide as well as selection of a material with suitable refractive index, which must be higher than the refractive index of the surrounding material, typically air.
Edge lit light guides for back lighting and front lighting of displays are well known, and they are inexpensive and robust.
Known edge lit light guides are designed to provide maximum uniformity of light output across the entire surface of the light guide.
FIG. 1 shows a schematic image of an edge lit light guide 10. The light guide comprises a waveguide material, such as a slab of solid material with a top face 10a, a bottom face 10b and lateral edges 10c. There are top and bottom edges which cannot be seen in FIG. 1, as the cross section of FIG. 1 is taken in the lateral side-to-side direction. The light guide is generally rectangular in plan view.
From the left side in FIG. 1, light is coupled in from a light source 12 and at the bottom of the light guide several light out-coupling (i.e. light extraction) features 14 (are placed. Light propagates under an angle θin inside the light guide with height H. The out-coupling features 14 in this example are drawn as half prisms with a half top angle α, height h, and a width w.
The light guide is formed as a dielectric slab made out of e.g. glass or polycarbonate. In the slab, total internal reflection at the borders keeps the light confined while the light propagates. The edges of the slab are typically used to couple in light and the small light out-coupling features 14 locally couple light out of the light guide. The light extraction is achieved when the light hits a surface with scattering properties, or else the light hits a surface that changes the angles with which light travels within the slab so that the incident angle becomes smaller than the critical angle (with respect to the normal).
In addition to LCD backlighting, light guides are now also being used in office luminaires as well as in outdoor lighting. These new applications have also meant a departure from the flat slab based light guides used in display applications.
Light guides are perfectly suited to spread light evenly over a large surface area, as is desired for the example of LCD backlighting.
However, light guides applied in LED based illumination systems also require beam shaping to keep glare at a sufficiently low level. Glare regulations such as the unified glare rating (UGR) and the so-called L65 performance (which is the luminance in cd/m2 of a luminaire at 65 degrees viewing angle with respect to the normal to the luminaire surface) are described by the EN-12464 specification (part 1 of which relates to lighting or indoor work places).
Creation of a low-glare beam with an angle cut-off in all directions is not possible using a single, light guide system and based on a non-collimated light input.
FIG. 2 shows the configuration of another example of light guide system as used in some luminaires and backlights. As in the example of FIG. 1, it has light extraction features 14, which in this example comprise screen printed or inkjet-printed paint dots on the outside of a clear light guide plate. Light is again injected at one or more edges (straight or curved) of the light guide.
A visually uniform light emitting surface can be optimized when the small paint dots (typically <1 mm) vary in size or density over the surface. However, light is extracted from the light guide in an inefficient way and escapes in all angular directions giving high glare. FIG. 2 also shows a reflector back plate 20 for recycling light which escapes from the wrong side of the light guide.
There is therefore a need for a light guide based lighting system which enables creation of a cut-off in all directions but does not require the in-coupling of collimated light.
US2002135996 discloses a planar light source device comprising a light source arranged at an edge of the light guide pipe comprising light extraction features, the light source device further comprising a reflection sheet arranged to redirect extracted light from the light guide back through across the light guide.